1. Field of the Invention
This invention relates in general to structural members and methods of forming structural members, and more particularly, to a composite structural member, such as a bridge unit, which has a relatively thin molded deck portion adaptable for use on original structures and refurbished structures.
2. Description of the Prior Art
In the prior art there are a wide variety of structural members, both prefabricated and fabricated in place. These structural members include single element members, such as steel beams, and composite structural members with molded materials reinforced with, or supported by, metal bars or structural beams, girders or other elements. A typical molded material for the deck portions of these structures is concrete.
In forming structural members for spanning between two supports, it has often been found desirable to utilize a steel structural support beneath a molded concrete deck surface. Because steel can withstand a much higher tensile strength, these composite structural members are formed with a steel sustaining most of the tensile stress which is placed on the composite member.
To form composite members of the type having an upper concrete surface and a metal support underneath, a metal piece form mold typically is utilized. First, the steel supports, such as wide flange girders, are placed beneath a mold assembly having two or more mold pieces disposed around the girder or girders. Next, the concrete is poured into the mold such that the concrete fills the mold and extends over the girder. When the concrete is hardened, the mold pieces are disassembled from around the girders such that the concrete rests on the girders. In most instances, these wide flange girder-supported concrete structural members are formed in place. This is usually advantageous so the concrete surface can better fit into the finished structure. The concrete deck portion is attached to the beams by shear connectors which are molded into the concrete, or which extend into openings in the concrete which are then grouted in place.
In such composite structures, the concrete deck portion must be sufficiently thick to support the load applied thereto. Such loads include the weight of the concrete itself and any external loads which are applied, such as traffic on a bridge. The volume of concrete in such structures makes them quite heavy, and the cost of building such structures is usually high.
There is a need, therefore, for a lighter weight, less expensive structure which provides at least strength characteristics which are at least as high as prior structures. The present invention utilizes a considerably thinner concrete deck portion but shows the same or better strength properties.
A problem with all composite structures is that they eventually need repair. Over time, the loading on the concrete tends to cause cracking and other damage. This is particularly true in applications where the loading varies, such as traffic flowing over a bridge structure. Eventually, the composite structure must be refurbished. In many cases, this requires removal of the original molded deck portion and replacement thereof. If the replacement deck is made in the conventional manner, by molding a new thick deck portion, the procedure is very time consuming. This is of particular significance in the refurbishment of structural members such as bridges because it is desirable to keep the interruption of traffic to a minimum.
The present invention can be built in place but includes an embodiment utilizing prefabricated composite units which can be positioned on the old girders and attached thereto much more quickly than the time necessary to rebuild the original type of thick deck portion. In addition, the composite units have a substantially thinner deck portion and yet provides greater strength so that the replacement structure is slightly stronger and lighter than the original.